maybe i've found a little error in your example.
when using the glut timer function, you must reset the timer callback inside the timer function.

----- old code ----
void ctimer(int value)
{
glutPostRedisplay();
}

------ new code ----
void ctimer(int value)
{
glutPostRedisplay();
glutTimerFunc(20, ctimer, 1);
}

with this trick the animation run smooth